Electrolytic refining of impure aluminum



Jan. 30, 1951 A. F. JOHNSON 2,539,743

ELECTROLYTIC REFINING 0F IMPURE ALUMINUM Filed Jan. s, 1946 4 Sheets-Sheet 1 ixxxwxmwxm 6 I N V E N TOR.

e PM, mmwwm ATTORNEYS Jan. 30, 1951 A. F, JQH 2,539,743

ELECTROLYTIC REFINING 0F IMPURE ALUMINUM Filed Jan. 3, 1946 4 Sheets-Sheet 3 NV E NTOR,

Jan. 30, 1951 JOHNSON 2,539,743

ELECTROLYTIC REFINING OF IMPURE ALUMINUM Filed Jan. 5, 1946 4 sheets-sheet 4 Q E Q 2:. I K @7719; l 1 vi; 1

Patented Jan. 30, 1951 ELECTROLYTIC REFINING F Ill [PURE ALUMINUM Arthur F. Johnson, Cambridge, Mass., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Application January 3, 1946, Serial No. 638,831

This invention relates to the refining of impure aluminum and has for its object the provision of an improved method and apparatus for the electrolytic production of aluminum from 3 Claims. (Cl. .20467) passing the partially purified aluminum into receiving vessels in the salt fusion of an electrolytic cell wherein pure aluminum is deposited in the fusion.

impure aluminum. My invention provides a These and other novel features of the invensimple and eilicient method of converting imtion will be better understood after considering pure aluminum, such as aluminum scrap, 0011- the following discussion taken in conjunction taming various alloying metals as impurities into with the accompanying drawings, in which: pure aluminum and with the recovery of other Fig. 1 is a plan view of electrolytic apparatus metals from the impure aluminum. My invenembodying the invention; tion is particularly concerned with the refining Fig. 2 is a sectional view along line 2-2 of of molten impure aluminum and provides a Fig. 1 method and apparatus for electrolytically de- F 3 is a Sect onal V ew alo e 3-3 0f positing the pure aluminum on a cathode in a Fig. 1; salt electrolyte fusion, Fig. 4 is a partial plan view of the cell of Fig. 1

In accordance with my method, I provide in a withthe cover removed; salt fusion several separate bodies of molten im- Fig. 5 is a sectional side View of a modification pure aluminum; each in a retaining vessel adof the cell of Fig. 1; vantageously having a porous diaphragm be- Fig. 6 is a view along line 6-5 of Fig. 5; and tween the adjacent bodies, and pass an electric Fig. 7 illustrates a combination of apparatus current through the fusion from one body to the embodying the invention. other to electrolytically transport the aluminum The embodiment of the invention illustrated through the diaphragms and deposit it on cathin Figs. 1 to 4 comprises a supporting concrete odes interposed between the bodies. I employ a structure 1 including a wall and a bottom in temperature above the melting point of alumiwhich is placed a steel lining 2 to serve as a num and the aluminum deposited on the cathmoisture and liquid barrier, and a layer of heat odes coalesces in drops and falls to the bottoms and electrically insulating material 3 such as of the vessels where it is accumulated for rerammed alumina. The inner lining 4 which is moval. In this embodiment of my aluminum repreferably formed of acid-proof bricks, such as fining method, I employ a number of separate red shale bricks, is in contact with the fused salt refractory electrically non-conducting vessels for electrolyte 5. The bath may be formed of any the bodies of impure molten aluminum which are suitable halid material, such as a mixture of pervious to the passage of aluminum ions. I pass aluminum chloride and potassium chloride. I the electric current through the several bodies of prefer to line the cell with vitreous dense bricks aluminum in the vessels in series and selectively which are substantially non-porous and insoluble separate the aluminum from the impurities in the fusion. I may coat the inner surface of which are retained in the vessels. the lining with an aluminum nitride'refractory, My invention also contemplates the provision such as described in my copending application of a scrap melting operation in combination with Serial No. 653,967, filed March 12, 1946, now Patthe electrolytic purification just described. Adent 2,480,475, dated August 30, 1949. The bottom 'vantageously, I may subject scrap aluminum to a of the cell receptacle for receiving the salt elecmelting operation at a temperature resulting in trolyte slopes from both sides towards the center a partial separation of impurities from the to the channel 6 which, in turn, slopes towards aluminum and pass the aluminum thus partly one side to a collecting sump 1 from which the purified to the retaining vessels in the fusion for pure aluminum may be pumped out through the further purification. upright duct 8. As best shown in Figs. 2 and -3, My invention also comprises, in combination, a two rows of bricks In and H project inwardly sloping hearth aluminum scrap melting furnace and provide supports for the series of deep narwith means for effecting a partial'separation of y row vessels l5 which are formed of porous re impurities from the aluminum;.and means for fractory material, such as porous fused alumina.

These vessels are for receiving the separate bodies of impure aluminum to be purified and are held in proper spaced relation with respect to each other by the fused alumina spacers It.

The removable cover assembly 26 comprises a flat metal plate 2| with three steel stiffening bars 22 attached thereto and several projecting arms 23 by means of which the cover assembly may be manipulated. The inside of the steel cover plate 2| is covered with a layer of insulating refractory 24, such as alumina, and the inner surface is covered with a layer of fire clay 25. The refractory lining is held in position by the angle member 26. Interposed between the horizontal flat surface of the angle member 26 and the upper surface of the layer of bricks a is a layer of asbestos packing 21 on which the entire cover assembly rests to effect the exclusion of air and its contained moisture from the inside of the cell and the loss of vaporized salt. Directly above each of the vessels l5, a hole 28 is formed through the cover and a short section of pipe 29 attached to the cover plate 2| extends upward a short distance and is covered with a removable cap 38. These holes, pipes and caps extend along the longitudinal center of the cell, as shown in Fig. 1. and provide a means for pouring molten impure aluminum into the porous vessels 95 from any suitable feed manifold having spouts insertable into the various holes.

A pair of identical graphite anodes 32 are con-- nected to the current supply bars 33 which in turn connect to the power line 34. Each anode has a circuit clamp 35 attached to the bar 33 and also attached to an annular bracket 35 which is welded to the cover plate 2|. A layer of electrical insulation 31 is interposed between the clamp 35 and bracket 36 to prevent current flowing into the cover plate. The inside of the clamp has an annular space for receiving the packing material 38, such as asbestos, which is held in gas sealing contact with the a odes by means of the gland member 39. The lower ends of the anodes are inserted into the first porous vessel of the series.

The identical graphite cathodes 4! are each mounted in a clamping means 12 similar to that which supports the anodes which are connected to the conducting bars 43 attached to the power line 44. The cathodes are suspended in the salt fusion close to one side of the last porous vessel of the series. Two thermocouples '25 and 46 for measuring the temperature of the fusion are inserted through the cover and into the fus on. The entire cover assembly comprising the refractory lining, the feed holes, anodes, cathodes and thermocouples is removable as a unit. The purpose of using two anodes and two cathodes is to get a better distribution of current through the series circuit.

The intermediate electrodes '3 are in the form of graphite slabs with a longitudinal cut by means of which they may be hung over the side of each porous vessel 5 with part of the graphite inserted into the bodies of molten aluminum 5| and part of the graphite in the salt fusion between the vessels.

In carrying out an operation of the invention in the apparatus of Figs. 1 to 4, the cell is heated and provided with the molten salt fusion 5. This fusion may be formed, for example, of a mixture of aluminum chloride 55% and potassium chloride 45%. The electrolyte is maintained at a temperature from the melting point of aluminum up to around 800 C. While various types of impure aluminum may be treated, such as scrap aluminum or original aluminum to be upgraded, the invention is especially effective in the purification of scrap containing iron, silicon and copper. A typical example of scrap aluminum which may be treated successfully has the following composition:

Percent Si .78 Fe 1.02 Cu 4.24 Mn .45 Mg .97 Cr .11 Ni .28 Ti .06 Pb 3 Zn .26 Sn .05 Al Balance The vessels [5 are filled with the impure molten aluminum through pouring spouts inserted through the holes 28 after removing the caps 29. During the operation, the filling holes are closed by the caps 30. An overall voltage between the anodes 32 and cathodes M of around 20 volts is satisfactory, giving approximately 2 volts between the intermediate electrodes when ten porous vessels are used. A cell of the type illustrated may be operated at a current density of around 2 amperes per square inch, especially when the cells are full of aluminum. The current density will, of course, vary as the metal level drops. During the operation, the aluminum ions are electrically transported through the porous walls of the vessels [5 and deposited as pure aluminum on that portion of the graphite intermediate electrodes suspended in the salt fusion. The metal coalesces in tiny droplets on the upright surfaces falling to the bottom of the cell and eventually flows into the channel 5 and the collecting sump 1. Periodically the cover is removed from the duct 8 and the pure aluminum is pumped out of the cell.

The metals usually present in scrap aluminum are so much lower in electromotive series that they are not transported through the porous fused alumina vessels l5 and accordingly remain in the vessels I5. These metals accumulate in the bottom and may be removed from time to time to prevent an undue accumulation. It will be apparent that these metallic impurities include several very valuable metals which are in easily recoverable form.

The modified form of cell illustrated in Figs. 5 and 6 is provided with anodes, cathodes and intermediate graphite electrodes similar to those illustrated in Figs. 1 to 3. The principal difference between the two cells is in the bottom construction and in the bottom shape of the porous fused alumina vessels.

The cell comprises a concrete exterior 68, a steel shell 6| serving as a barrier, rammed insulating refractory 62, and an inner layer of dense refractory brick, such as red shale brick 63. Each of the porous fused alumina vessels 64 has a sloping or V-shaped bottom. The sloping sides of the bottom rest on two rows of bricks 8B and 61 projecting from two opposite sides of the cell. The bricks forming the sloping surfaces and H immediately beneath porous vessels are set very close, for example, around 1 inch or less from the bottoms of the vessels. The purpose of 7 this narrow space is to provide the smallest possible channel for the flow of current in the fusion under the porous vessels. The sloping surfaces 18 and H converge towards the channels 12 and 13 which in turn slope into the channel 14. The metal accordingly flows into the low end of channel 1'4 from which it may be pumped out by inserting a pump through duct 15.

The combination of apparatus illustrated in Fig. 7 comprises a sloping hearth scrap aluminum refining furnace 80 connected by conduit 8| -*to the electrolytic refining cell 82. The furnace has feed doors 83 and 84, preferably opening on a level with the sloping hearth 85 which slopes towards the metal receptacle or well 86. A rather large combustion space 81 is provided above the hearth and receptacle in which fuel is introduced through the burners 88. The fuel is preferably directed against an arched roof and the products of combustion pass through the stack 89. The molten aluminum runs from the hearth into the receptacle, and when the valve 90 is open, the metal flows through conduit 81 and down through the upright ducts 9| into the confining vessels92 of the electrolytic cell. The electrolytic cell 82 is similar to that shown in Figs. 1 and 2.

In carrying out an operation in the combination of apparatus illustrated in Fig. 7, scrap aluminum, such as scrap containing some iron and copper, is thrown through the doors 83 and 84 onto the hearth 85. A temperature'is used in the furnace such as to melt the aluminum without melting the iron and copper. The molten aluminum flows from the hearth 85 into the receptacle 8'5 and the metals which do not melt remain on the hearth. From time to time, these may be raked off the hearth through one of the doors 83 or 84. 7

That section of the conduit 8| immediately connected to the valve 90 is provided with an electrically heated jacket 93 which keeps the valve and duct thereunder at a temperature above the melting point of aluminum. At any time it is desired to replenish the metal in the vessels 92, the valve 98 may be opened to admit the proper amount of aluminum. The metal entering the electrolytic cell is in a semi-purified condition since much of the impurities were left in the sloping hearth furnace. If desirable, several electrolytic cells may be connected to the sloping hearth furnace and the operation so carried out that the aluminum flows 6 more or less continuously into the electrolytic cells.

I claim:

1. In the electrolytic production of pure aluminum from impure aluminum, the improvement which comprises forming in an electrically nonconducting vessel, a molten salt fusion maintained at a temperature above the melting point of aluminum, confining in the fusion at least one body of molten impure aluminum, electrolytically transporting aluminum ions from the body of impure aluminum through a porous alumina vessel to a cathode and collecting molten aluminum thereon, said collected aluminum falling from the cathode to the bottom of the vessel, and leaving in the body of aluminum metals like manganese, iron and copper which are sufiiciently below aluminum in the electromotive series that they are not transported through the porous alumina vessel to the cathode.

2. In the method or claim 1, using a salt fusion comprising aluminum chloride and another salt.

3. In the method of claim 1, providing in the salt fusion several porous alumina vessels each of which contains a body molten impure aluminum and transporting the aluminum ions from each body through the porous vessel to a separate cathode.

ARTHUR F. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 322,169 Farmer July 14, 1885 641,276 Darling Jan. 16, 1900 673,364 Hoopes Apr. 30, 1901 1,255,197 Malm Feb. 5, 1918 1,299,947 Ingeberg Apr. 8, 1919 1,545,383 Ashcroft July 7, 1925 1,545,384 Ashcroft July 7, 1925 1,833,806 Weber et al Nov. 24, 1931 1,854,684 Brode et al. Apr. 19, 1932 2,234,967 Gilbert Mar. 18, 1941 FOREIGN PATENTS Number Country Date 312,598 Great Britain May 27. 1929 

1. IN THE ELECTROLYTIC PRODUCTION OF PURE ALUMINUM FROM IMPURE ALUMINUM, THE IMPROVEMENT WHICH COMPRISES FORMING IN AN ELECTRICALLY NONCONDUTING VESSEL, A MOLTEN SALT FUSION MAINTAINED AT A TEMPERATURE ABOVE THE MELTING POINT OF ALUMINUM, CONFINING IN THE FUSION AT LEAST ONE BODY OF MOLTEN IMPURE ALUMINUM, ELECTROLYTICALLY TRANSPORTATING ALUMINUM IONS FROM THE BODY OF IMPURE ALUMINUM THROUGH A POROUS ALUMINA VESSEL TO A CATHODE AND COLLECTING MOLTEN ALUMINUM THEREON, SAID COLLECTED ALUMINUM FALLING FROM THE CATHODE TO THE BOTTOM OF THE VESSEL, 